1. Field of the Invention
This invention relates to a method for controlling a carbon baking furnace. The invention is particularly concerned with so-called ring furnaces, commonly used in baking carbon bodies to form the anodes for use in electrolytic production of aluminium.
2. Description of the Prior Art
The quality of the anodes seriously affects the cost of production of aluminium. Therefore, the baking process is, in terms of cost, one of the key concerns in aluminium production. Within the anode production process, the baking, which constitutes about 70% of the total cost, represents by far the most expensive stage. Except for capital costs, which are mainly caused by the furnace, all other costs, i.e. about 70%, are immediately affected by the firing and control system of the furnace. Depending on potline operating cost and anode cost, an economical balance has to be found between two opposing objectives.
High anode quality causes higher coke cost, shorter refractory lifetime, and higher energy cost, while low anode quality causes higher electrolysis cost. A specific combination of all parameters can be found, which leads to the lowest overall cost. This cost-minimum defines a specific anode quality. The task of the carbon plant is to produce this defined anode quality. The baking costs for coke, refractory and energy are effected by the average of anode quality. But for the electrolysis the average of the anode quality is not the only important parameter. If the portion of xe2x80x9cbadxe2x80x9d anodes exceeds a specific value, the whole electrolysis suffers. Therefore, the same portion of bad anodes can be achieved at a lower average of quality, i.e. at lower baking cost, if the consistency of the anode quality is increased.
The consistency is affected by the furnace design, the firing equipment, and the control philosophy. This invention is focused on an advanced control philosophy to increase the anode quality consistency and the flexibility in terms of a most efficient combustion.
The anode bake quality is defined by the heat treatment. Each anode has to be brought to a specific temperature for a specific time. More precisely, the quality of the anode is determined by the time during which the body is kept at a temperature above the sintering temperature. As a parameter to indicate the quality, a time integral of the temperature above this sintering temperature can be defined as the xe2x80x9cbaking indexxe2x80x9d.
Ring furnaces have been used for many years in the baking of carbon anodes. These furnaces (FIG. 1) include a cooling zone 100 having five adjacent cooling sections 101, 102, 103, 104 and 105, a firing zone 200 having four adjacent firing sections 201, 202, 203 and 204, and a pre-heat zone 300 having three pre-heat sections 301, 302 and 303. Air at ambient temperature enters the furnace flue system through a cooling inlet, such as a manifold 1, and flows through the flues in a tortuous path as indicated by the curved arrows. Manifold 1 is shown at the entry of cooling section 105, but a further inlet may be provided at subsequent cooling sections in the fire direction. Firing frames 2, 3, 4 and 5 are provided for the infection of gaseous or liquid fuel into the firing sections. The flue gases leave the furnace through an exhaust outlet such as manifold 6. The general direction of forward movement of the flue gases is referred to as the direction of the fire and is indicated by arrow 10.
Green anodes initially charged into the furnace pits are progressively treated by changing the heating/cooling conditions by moving the exhaust manifold 6, the gas (or other fuel) injection firing frames 2, 3, 4, 5, and the cooling manifold 1 progressively in the xe2x80x9cdirection of firexe2x80x9d 10, as illustrated in FIG. 1. The draft within the flues is initially provided by air which enters through the cooling manifold 1. The cooling manifold 1 preferably injects air under positive pressure into the last cooling section 105 and the air flows forward either under its own positive pressure or because it is drawn forward by the negative pressure exerted by the exhaust manifold 6 at the other end of the active zone of the furnace. The draft through the flues is controlled so that there is negative pressure within the flues in at least the firing or baking sections 200 and in the preheat sections 300.
The air which initially enters is cold and has at least its normal oxygen content. The temperature of the cooling air increases as it moves through the cooling sections 100 towards the firing sections 200, due to heat transfer from the anodes in the adjacent pits, thus progressively cooling the baked anodes. The air reaching the firing sections 200 is thus elevated in temperature to such a degree that it will support the combustion of gaseous fuel injected into the firing sections 200 through the firing frames 2, 3, 4, 5 connected to each such section 204, 203, 202 and 201. If a liquid fuel is used, the temperature of the incoming air is such as to support both the combustion and vaporization of the fuel. The temperature of the flue walls in the latter firing sections 204 (that is the rearmost firing section in the direction of the fire) may be raised to approximately 1225xc2x0 C. by combustion of the incoming fuel in the relatively high oxygen content incoming air in the second firing section 202, the oxygen content of the forwardly moving flue gases has been reduced and the temperature within the flue will also be lower than that in the third and fourth firing sections 203 and 204. The temperature and oxygen content of the flue gas falls further in the first firing section 201, so that the temperature of the flue gases leaving the first firing section 201 may have fallen to about 1000xc2x0 C.
In the preheat sections 300 the temperature of the unfired anodes is progressively raised by the hot flue gases which have a relatively low oxygen content after much of the oxygen has been used in the combustion process of the firing sections 200. However, as the temperature of the unfired anodes is progressively raised, volatile materials in the pitch, which are used to bind the carbon material forming the anodes together, are released and burn in the residual oxygen of the flue gases. The temperatures of the flue walls in the first preheat section 301, where the unfired anodes are first subjected to the heated flue gases, may be in the range 200 to 500xc2x0 C. In this first preheat section 301, all the heating of the unfired anodes takes place by extraction of the residual heat from the flue gases.
In the second preheat section 302, the temperature may rise to between 500 and 800xc2x0 C., and the anodes are heated both by the incoming flue gases and the combustion of the pitch volatiles which are driven off as the anode temperatures are raised. In the third preheat section 303 of the illustrated embodiment, the flue wall temperature may reach 800 to 1000xc2x0 C. due to the combined action of the incoming flue gases and the combustion of further pitch volatiles.
The flue gases are removed via the exhaust manifold 6 after passing through the flues of the first preheat section 301. The furnace section 401 preceding the first preheat section is packed with unfired anodes after the fired anodes from the previous pass of the fire have been unloaded from that section. The section 401 packed with unfired anodes then becomes the first preheat section when the manifolds and firing frames are next moved forward.
The condition of the flue gases in any active zone of the furnace can be controlled by adjustment of the amount of air supplied through the cooling manifold and extracted through the exhaust manifold, as well as by the amount of fuel gas injected into each firing section.
Each pit section is subjected in use to different heating conditions in order to bake green or unbaked carbon bodies into a desired anode material. The condition in each section is altered progressively between controlled limits until an upper limit is reached.
After a predetermined time, the major factors causing progressive alteration of the conditions in each section are moved to the adjacent downstream section. This type of movement takes place along the length of the furnace in which the carbon bodies are baked, and is referred to as fire move in the fire (downstream) direction. Thus, the carbon bodies do not move through the furnace but the furnace conditions are intermittently and progressively changed in each successive pit section so that any one section passes through a packing stage (not shown), to the several preheating stages 301, 302, 303 in which the temperature of the bodies is progressively increased, to several firing stages 201, 202, 203 and 204 in which the temperature of the bodies is progressively further increased, to several cooling stages 101, 102, 103, 104 and 105 in which the temperature of the bodies is progressively decreased, and finally to an unloading stage (not shown).
Any given section will have adjacent on one side a section which is being treated at the stage last followed by the given section, and on the other side a section which is being treated at the stage next to be followed by the given section. The successive sections under treatment together make up an active zone of the furnace and each furnace will normally have sufficient sections for at least two active zones, so-called fires to be operated at the same time.
The heating or cooling of the carbon bodies at the various stages is, as indicated above, brought about by heat transfer through the flue walls 1 from the gases 5 flowing through the flues 4. The flue gases are removed before a section in which the carbon bodies are initially packed into the furnace, so this section has no flow of gases through its adjacent flue walls. However, after this section, the carbon bodies are heated in the successive preheating stages 301, 302, 303 by flue gases which have passed through the firing stages 201, 202, 203 and 204. Following the preheating stages, the carbon bodies are baked by heat from the flue gases containing fuel gases which are burned to increase the temperature of the firing stages. After the firing stages 201-204, the carbon bodies are cooled in the successive cooling stages 101-105 by an incoming air flow which enters the flues at the final cooling stage 105.
In the preceding paragraph, the operation of the ring furnace has been described with reference to the treatment of the carbon bodies which are being baked in the furnace. However, as the carbon bodies are not themselves moved from section to section of the furnace, it is more conventional to consider the sections in relation to the air and fuel gas flows and the heat which is generated or removed in each section. When considered in this manner, the construction and operation of the furnace by movement in the fire direction can be seen to include a succession of preheat sections in which the green carbon bodies are progressively raised in temperature, a succession of firing or baking sections in which the temperature is progressively raised further, and a succession of cooling sections in which the temperature is progressively lowered. The flue gases move in the fire direction 10 from the cooling sections in at least the coldest of which cooling and combustion air enters the flues and moves towards the other end of the group of sections within which the action of heating, baking, and cooling the carbon bodies takes place. As the air which is the initial flue gas moves from the first cooling section, it is raised in temperature as it approaches the first firing section. In each successive firing section, fuel such as natural gas or fuel oil (hereinafter xe2x80x9cfuelxe2x80x9d), is injected into the flues where it burns in the incoming hot air stream.
As the fuel combustion proceeds, the oxygen content of the air stream is decreased and the temperature of the successive firing sections also decreases. After the last firing section, combustion continues in the flues with the burning of volatile materials, which are driven from the carbon bodies and the pitch used to bind them. These volatile materials (referred to as xe2x80x9cvolatilesxe2x80x9d) migrate through the flue walls to burn with the remaining oxygen in the air stream. In the last of the preheat sections, most of the residual heat is removed from the low oxygen air stream and serves to raise the temperature of the carbon bodies towards the temperature at which volatiles will be driven from them.
As the combustion of the fuel added to the furnace in the heating sections and of the volatiles driven from the carbon bodies in the preheat sections depends on the amount of oxygen available, control of the oxygen content of the gas stream drawn through the flues by the exhaust outlet is an important factor in maintaining the desired conditions throughout the furnace. The primary control over the draft is achieved by adjustment of the negative pressure (so called draft) exerted at the exhaust outlet. Increased draft draws more air into the flues and increases the amount of oxygen available over a given time at any point in the flue. However, the oxygen content of the flue gases is affected not only by the cooling and combustion air admitted at the end cooling section, but also by ambient air which may leak into the furnace due to imperfect sealing of various access apertures, such as inspection ports and fuel inlets, or through cracks in the furnace refractory.
In order to balance the numerous factors which affect the furnace operation and to make adjustments which will optimize both the furnace operation and the product quality, control systems have been devised which analyze data continuously or intermittently derived from observation of various furnace conditions. The analysis of the data is used to actuate appropriate control mechanisms which alter the furnace conditions towards predetermined optimum conditions.
Control systems are commonly computer operated. Various methods and apparatuses have been proposed to assist in the control of the furnace conditions and thereby produce the desired control over the quality of the baked products.
In International Patent Application PCT/FR87/00213 of Aluminium Pechiney, the gas flow rate in each flue is adjusted by the use of flap valves on each suction nozzle of the exhaust manifold. The flap valves are responsive to fluctuations in temperature and draft measurements in the flue and to capacity measurements made on the smoke from each flue.
In U.S. Pat. No. 4,354,828 assigned to South Wire Company and National Steel Corporation, the flue and pit temperatures are measured to produce a control signal which operates valves varying the air/fuel mixture of each burner in the baking stages of the furnace. This control mechanism is used to adjust the temperature in each flue for which the burner mixture is controlled.
In International Patent Application PCT/WO91/19147 a method for control of the oxygen/fuel ratio in a carbon baking furnace by obtaining the oxygen level in one section of the furnace by measurement or inference, and using this level to determine any changes in at least one of the flue gas flow rate, fuel injection level and air injection level, is described.
As referred earlier, a specific temperature vs. time profile in the flues surrounding the related anode pit is required in order to achieve a defined baking index. In the plants which are in service, a temperature vs. time schedule is specified giving the setpoints for the plant control units, following to the schedule as precisely as possible.
As shown in the description of the furnace operation, due to the amount of flue gas volume, bearing the oxygen, in relation to the total amount of fuel in the flue, consisting of the injected fuel in the fire sections and of the volatiles evaluated in the preheat sections, flooding situations occur, which prevent following precisely the temperature schedule. In addition, due to the fire move procedures, which last for more or less time, interruptions and deviations of the firing profile during the entire baking process are inevitable. But any deviation causes a permanent lack or excess of heat flow to the anode and so a dispersion of anode quality. Therefore, a fundamental limitation of the anode quality consistency is given with the conventional control philosophy, even with an accurate control system.
The above prior proposals are not entirely satisfactory and it is an object of the present invention to improve the control of carbon baking furnaces, thus leading to improved product quality and consistency.
Furthermore, in respect to an optimum operation of the furnace regarding fuel consumption and emissions of unburned volatiles, it is a further object of the invention to obtain the desired anode quality without having to follow a rigid temperature schedule at any time.